When optical signals (pulses) are transmitted via an optical fiber, they are distorted. This distortion reduces the signal quality and considerably restricts the range without regeneration, especially at high bit rates. Wavelength-dependent attenuation can be compensated for via suitable amplifiers. Suitable measures are used in an attempt to compensate for other effects such as group delay time dispersion, polarization mode dispersion (PMD) and self phase modulation (SPM).
A method for PMD compensation via an optical first-order compensator and by via an adjustable electrical transversal filter are compared in “Optics Communications”, Volume 182, No. 1-3, pages 135-141.
Until now, compensation for group delay time dispersion, which is referred to as dispersion for short in the following text, mainly has been carried out by using special fibers whose group delay time characteristics are the inverse of those of the transmission fibers.
An adjustable electrical transversal filter which is used to compensate for PMD and dispersion is described in “ECOC'99” Vol. 2, pp. 138-139, H. Bühlow et al.
The International Patent Application WO 00/50944 discloses an arrangement for dispersion compensation in which a Bragg grating filter is controlled as a function of the dispersion. By way of example, a dispersion analyzer assesses the eye opening or the bit error rate. The determined signal quality is used as a closed-loop control criterion. The arrangement likewise can be used for PMD compensation.
European Patent Application EP 0 740 173 A2 discloses the use of an optical transversal filter for dispersion compensation in individual channels, or for all channels at the same time, in a wavelength-division multiplexed system. This reduces the complexity in comparison to separate filters or compensation arrangements for individual channels. Compensation via a periodic filter has the same compensation effect for all channels but is not optimum in the presence of wavelength-dependent dispersion (higher-order dispersion), in which the gradient of the dispersion profile changes.
Furthermore, it is known, per se, for dispersion fundamental compensation to be carried out via an appropriate fiber, and for this to be combined with fine compensation using electrical filters.
An object of the invention is to derive an arrangement for adaptive signal equalization.